Apologies if this is not the right forum page/method of posting for this or the assets turn out not to be useful to people here, but through a game design course that I am doing on Udemy, I have found out that the makers of the (now defunct) multiplayer game Glitch have released all their game assets (and client/server code) for free under the Creative Commons CC0 1.0 Universal License. They are also making their Slack collaboration tool available.